Shoppers hit the nation’s stores yesterday armed with gift cards that could be the missing ingredient for a happy holiday season in retailing.

The day after Christmas is traditionally a busy time for stores, and this time was no exception.

Hundreds of eager shoppers lined up at Macy’s Herald Square for the doors to open to savings of as much as 75 percent.

Many of them clutched gift cards, which Macy’s and other retailers hope will boost what otherwise could become a lackluster Christmas.

Saks and Bergdorf Goodman also offered savings of up to 70 percent.

Experts say that as many as one out of five consumers go bargain hunting in the post-Christmas buying binge, the period that’s now regarded as make-or-break for holiday sales.

Gift cards – an increasingly popular gift for savvy shoppers – are producing even bigger chunks of overall sales, as much as 8 percent of the year’s business, says Eileen Tolly, a spokesperson for the National Retail Federation. She says it’s about $17.24 billion in added sales from gift cards alone that get redeemed between now and in January.

Other retail experts say gift cards produce as much as 15 percent of sales. Most retailers don’t record revenue from gift cards until the card is used to make a purchase.

Analysts say retailers this year are depending more on gift card roulette than ever.

Michael Niemira, the chief economist for the International Council of Shopping Centers, estimates that 35 percent of gift cards are redeemed within the first two weeks.

Although the estimates vary, gift cards are expected to account for at least 10 percent and perhaps as much as 15 percent of overall holiday spending this year, up considerably from last year.

“The week after Christmas has become an important ‘Act II’ of the holiday season,” noted Sears, Roebuck & Co. Retailers likely to benefit most from gift cards are the ones that have already recorded strong sales this season, analysts said.